1. Field of the Invention
The present invention relates to an amplifier circuit, and more particularly, to a low-noise amplifier circuit typically used for a wireless communications equipment.
2. Description of the Related Art
In a radio frequency (RF) signal receiving apparatus such as a cellular phone and a base station of a wireless communication system, a received signal has very weak intensity and includes considerable noise mixed therein. Accordingly, such a signal receiving apparatus requires a circuit for amplifying the received signal while reducing a noise figure of the signal. A low-noise amplifier, which is typically installed in an input stage of the signal receiving apparatus, amplifies the input RF signal so that an amplified signal has a required gain and noise figure.
As illustrated in FIG. 1, a conventional low-noise amplifier 10 includes an amplifying circuit 14 performing an amplification of the RF signal and impedance matching circuits 12 and 16 for matching impedances between the amplifying circuit 14 and external circuits. The input and output impedance matching circuits 12 and 16 are designed based on scattering parameters (S-parameters) so that reflection coefficients are minimized at input and output stages of the amplifier. Meanwhile, since the low-noise amplifier determines the overall noise characteristics of the signal receiving apparatus, the amplifier is usually designed in a balanced type rather than a single-ended type so as to have a minimum noise figure.
As shown in FIG. 2, a balanced type low-noise amplifier includes at least one amplifying stage, a power supply for providing DC power to the amplifying stage, hybrid couplers for splitting or combining signals at input and output stages, delay compensating circuits for compensating phase difference between the signal paths existing between the hybrid couplers, a noise removing circuit for reducing noise, and a comparator for comparing a signal or a supply voltage with a reference. In particular, the power supply can be implemented in various manners to procure an optimum amplification.
If any one of the internal circuits of the amplifying circuit happens to be damaged or the supply voltage is lost, the amplifying circuit cannot operate properly. In such a case, the signal receiving apparatus or the overall communication system happens to be faced with a significant signal loss. Meanwhile, when the low-noise amplifier cannot operate normally, the comparator in the amplifying circuit may detect the abnormality to notify a user via a light emitting diode so that the user replaces or troubleshoots the amplifying circuit. However, in a critical situation, the system may be inoperable at all until the user or a maintenance personnel replaces or troubleshoots the amplifying circuit.